The present invention concerns exposure-duration control circuits for photographic cameras, of the digital type, in which a digital counter counts pulses from a pulse generator, the pulse generator including a photosensitive element exposed to scene light and furnishing a pulse train whose pulse repetition frequency varies in dependence upon the scene light to which the photosensitive element is exposed. When the light-totalizing counter has reached a predetermined count, a terminate-exposure signal is generated, e.g., to deenergize a shutter-holding magnet and permit the camera's shutter to abruptly close. The circuit typically includes a start switch which resets the counter earliest upon completion of an exposure and/or also serves to initiate counting at the start of the exposure. The light-dependent pulse generator may include a photosensitive element and a timing capacitor connected in circuit with a threshold circuit stage to form an oscillator.
Such an exposure-duration control circuit is disclosed in Federal Republic of Germany published allowed patent application DE-AS 22 19 523. The duration of the exposure is established by the number of pulses which the light-totalizing counter must count before generation of the terminate-exposure signal. In low-light conditions, there is the disadvantage that the shutter may remain open for excessively long times. If the shutter is held open by a shutter-holding magnet which is deenergized to terminate the exposure, then this can, for example, result in excessively protracted energization of the shutter-holding magnet, leading rather quickly to camera-battery discharge.
With exposure-duration control circuits of other types, it is known, when a flash exposure is to be made and the camer's flash unit is switched on in readiness for operation, to place an upper limit upon the duration of the exposure by limiting the maximum value of the exposure duration which the photosensitive element of the timing circuit is capable of establishing. For example, it is known to connect a fixed resistor parallel to the photoresistor or else to connect it instead of the photoresistor in series with the timing capacitor of the circuit.